In July, 1959, J. Dyson published a paper (Journal of the Optical Society of America, 49, 713) disclosing a self-conjugate optical system having certain interesting properties. The present invention relates to modifications and improvements of the Dyson system.
To establish the general context of the present invention and to make clear the departures from prior-art practice, FIG. 1 of the drawing depicts the basic design of the Dyson system. As shown in FIG. 1, the Dyson system is essentially comprised of a concave spherical mirror M having a center of curvature C and a plano-convex lens L. The planar front surfaces FS of the plano-convex lens L is coincident with the center of curvature C of the spherical mirror M. The spherical back surface BS of the lens L is concentric with the spherical mirror M. The spherical mirror MS is located coincident with the paraxial back focal point BFP of lens L.
According to the Dyson principle, if the refractive index of the lens is n, and if the radii of the spherical back surface BS of the lens L and of the spherical mirror M are r.sub.L and r.sub.M, respectively, then r.sub.L and r.sub.M are to be related by the equation EQU r.sub.L = (n - 1)/n .multidot. r.sub.M.
because of the concentricity of the spherical surfaces BS and M, the Dyson system is self-conjugate, with image and object planes coinciding with the planar front surface FS of the lens L.
Dyson explains that the concentric relationship of the spherical surfaces in conjunction with the relationship between r.sub.L and r.sub.M indicated above causes the seven Seidel aberrations to become zero.
Dyson also explains that the sagittal field for the system is flat to all orders. However, Dyson notes that higher-order tangential astigmatism in his system is of such a character as to markedly limit the size of the useful field. This tangential aberration is proportional to the fourth power of the size of the field (the distance from the optical axis of the system) and is convex to the planar front surface FS of the lens L (overcorrected).
FIG. 2 of the drawing illustrates the character of this tangential aberration in the Dyson system. The tangential focal surface T represents the loci of the object points of the system produced by tangential rays. Plotted along the horizontal axis are the distances of such tangential-field object points from the planar front surface FS of lens L. Plotted along the vertical axis are the distances of the object points from the optical axis OA of the Dyson system. It will be noted that the tangential aberration in question is zero at the optical axis, is initially of low value as one proceeds away from the optical axis, and then begins to increase more and more steeply as one proceeds still further away from the optical axis. As indicated above, this tangential aberration is proportional to the distance of the object point from the optical axis, raised to the fourth power. Also to be noted is that the tangential field is overcorrected for all field sizes, i.e., is always located in the space in front of the planar front surface FS of the lens, and is at no point undercorrected, i.e., is at no point located within the material of lens L behind the planar front surface FS of the lens.
Dyson points out implicitly that the spherical mirror M constitutes the system stop. Because the mirror M is located, in accordance with the equation presented above, at the paraxial back focal point BFP of the lens L, the chief rays entering the planar front surface FS of the lens are all parallel to the optical axis OA, in so far as third-order aberrations are concerned. However, the spherical back surface BS of the lens produces spherical aberration in the bundle of chief rays, so that a representative chief ray emerges from the system at the planar front surface FS at an exit angle different from its entrance angle; the aberration depicted in FIG. 2 of the drawing is a result of this. The severity of this tangential aberration led Dyson to express the belief that his system was restricted to small field size, albeit large apertures.